HDU 2197 本原串

快速幂+找规律

#include<cstdio>
#include<cstring>
#include<cmath>
#include<map>
#include<algorithm>
using namespace std;
map<int,int>C;
const int MOD=2008;
int n;

int fast_power(int a,int b)
{
    int c=1;
    while(b!=0)
    {
        if(b%2==1) c=(c*a)%MOD,b--;
        else a=(a*a)%MOD,b=b/2;
    }
    return c;
}

int cal (int x)
{
    int ans,i;
    if(C[x]!=0) return C[x];
    else
    {
        ans=fast_power(2,x);
        for(i=1;i*i<=x;i++)
        {
            if(i==1) {ans=(ans+MOD-cal(1))%MOD;continue;}
            if(i*i==x&&i!=1) ans=(ans+MOD-cal(i))%MOD;
            if(i!=1&&i*i<x&&x%i==0)
            {
                ans=(ans+MOD-cal(i))%MOD;
                ans=(ans+MOD-cal(x/i))%MOD;
            }
        }
    }
    return ans;
}

int main()
{
    C.clear();
    C[1]=2;
    while(~scanf("%d",&n))
    {
        if(C[n]==0) C[n]=cal(n)%MOD;
        printf("%d\n",C[n]);
    }
    return 0;
}